Drive devices having at least two switchgears actuated in closing direction by spring force and actuatable in opening direction via hydraulic pressure are often used to drive construction machines like mobile excavators. A hydrostatic engine preferably drives here the drive device in which two speed ranges can be selected by switching the first or second switchgear. Both switchgears are closed in pressureless state by spring force and are thus preferably interconnected via a planetary gear so that in this state the output is non-rotatably held and thus the switchgears are actuated in the manner of a parking brake.
DE 198 20 903 A1 discloses a drive device having two hydraulically switchable friction clutches actuatable via hydraulic pressure in opening direction and via spring force in closing direction and which in pressureless state act as parking brake. When the parking brake is actuated it is possible, by manual reduction of a store space, to build up a hydraulic pressure which can be supplied via two valves to both switchgears and thus simultaneously actuates said two switchgears in opening direction in order to make possible towing the vehicle, for example, in case of breakdown failure. In said drive device, via said valves, pressure medium from the storage space arrives at the switchgears and thus mixing pressure medium with said space can soil the switchgear. The storage space has to be entirely filled with pressure medium which presupposes a costly ventilation of the storage space. To protect the drive device from overpressure, the storage piston has a relief valve which communicates with a tank via a hose pipe. The drive device thus has to be connected via an additional hose pipe with the pressure medium tank.
The problem on which this invention is based is to provide a drive device where two switchgears, which are actuated in closing direction and thus non-rotatably keep the output in the manner of a parking brake and easily actuate it in opening direction, make possible moving the vehicle even when the prime mover is stationary and thus no pressure medium supply exists.